Pneumatic tires typically have a circumferential rubber tread with an outer running surface intended to be ground contacting. The tire tread may be of a cap/base construction comprised of an outer rubber tread cap layer having a running surface and an underlying rubber tread base layer. Such tread cap/base layered construction is well known to those having a skill in such art.
In practice, the tread intended to be ground contacting (containing the running surface) may be comprised of an electrically resistive, or insulating, (poorly electrically conductive) rubber composition and said tread base layer comprised of a relatively electrically conductive rubber composition (relative to the rubber composition of said tread cap layer). Such electrically resistive tread may, for example, electrically resistive in the sense of a silica reinforced rubber composition which contains less than about 25 parts by weight per 100 parts by weight rubber (phr) of rubber reinforcing carbon black.
In such instance, it may be desired to provide a path of least electrical resistance through such electrically resistive tread to the running surface of said tread cap layer to enable an electrical discharge of electrical potential generated by the associated vehicle through the wheel on which the tire is mounted.
For, example, numerous proposals involving a physical path of least electrical resistance has been may be provided through or around an electrically resistive rubber tread to its running surface by an electrically conductive rubber strip containing, for example, at least 40 parts by weight per 100 parts by weight (phr) of rubber reinforcing carbon black. Exemplary of such proposals, which is not intended herein to be limited or all-inclusive, is for example, U.S. Pat. No. 5,942,069.
For this invention, an improvement, which is intended to be a significant departure from such past practice, is an inclusion of an ionic compound, particularly an ionically conductive ionic compound, in the otherwise electrically resistive tread rubber composition which contains, for example, less than 25 phr of rubber reinforcing carbon black. It is contemplated that such inclusion would obviate use of a physical insertion of a an electrically conductive rubber strip through or around the tread for such purpose. In this manner, then the tread itself becomes the path of least electrical resistance to its outer running surface without necessitating and therefore to an exclusion of an electrically conductive strip through or around the tread itself to its running surface.
While the mechanism might not be entirely understood, it is contemplated than an inclusion of the ionic compound in a from of an ionically conductive ionic compound within such tread rubber composition acts to provide a path of least electrical resistance by permitting a migration of electrical potential generated ions within the ionic compound toward the running surface of the tread and thence to the ground which contacts the tread running surface.
Further, while the mechanism might not entirely understood, it is alternatively contemplated that an inclusion of the ionic compound in a rubber composition which contains reinforcement in a form of a combination of precipitated silica and rubber reinforcing carbon black can act to improve one or more physical properties of the rubber composition such as, for example, one or more of Shore A hardness, hot rebound and tan delta properties.
In the description of this invention, the term “phr” relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated. The term “carbon black” is used to refer to rubber reinforcing carbon blacks unless otherwise indicated. Exemplary rubber reinforcing carbon blacks may be referred to, for example, in the Vanderbilt Rubber Handbook (1978) on Pages 414 through 417.